Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
Monoethylene glycol is typically prepared from ethylene oxide, which is in turn prepared from ethylene. Ethylene and oxygen are passed over a silver oxide catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 weight percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a recirculating solvent stream containing mostly water. The ethylene oxide-depleted stream is partially or entirely supplied to a carbon dioxide absorption column wherein the carbon dioxide is at least partially absorbed by a recirculating absorbent stream. Gases that are not absorbed by the recirculating absorbent stream are recombined with any gases bypassing the carbon dioxide absorption column and are recycled to the ethylene oxide reactor.
The solvent stream leaving the ethylene oxide absorber is referred to as fat absorbent. The fat absorbent is supplied to an ethylene oxide stripper, wherein ethylene oxide is removed from the fat absorbent as a vapour stream. The ethylene oxide-depleted solvent stream is referred to as lean absorbent and is recirculated to the ethylene oxide absorber to absorb further ethylene oxide.
The ethylene oxide obtained from the ethylene oxide stripper can be purified for storage and sale or can be further reacted to provide ethylene glycol. In one well-known process, ethylene oxide is reacted with a large excess of water in a non-catalytic process. This reaction typically produces a glycol product stream consisting of almost 90 weight percent monoethylene glycol, the remainder being predominantly diethylene glycol, some triethylene glycol and a small amount of higher homologues. In another well-known process, ethylene oxide is reacted with an equimolar amount or slight excess of water in the presence of a hydrolysis catalyst. In another well-known process, ethylene oxide is catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate can improve the selectivity to monoethylene glycol.
Most conventional processes comprise steps of converting ethylene to ethylene oxide, absorbing ethylene oxide from a gas stream into a liquid stream and subsequently reacting ethylene oxide to ethylene glycol. GB 2 107 712 describes a process wherein the absorption and reaction steps are combined: gases from the ethylene oxide reactor are supplied directly to a reactor wherein ethylene oxide is converted to ethylene carbonate or to a mixture of ethylene glycol and ethylene carbonate.
The ethylene glycol-containing solutions that result in the known processes are subjected to water removal, typically in a series of flashing and/or distillation steps. The water removal can be an energy intensive process, particularly if a large excess of water is present in the ethylene glycol-containing solution. Processes wherein the absorption and reaction steps are combined provide ethylene glycol product solutions containing particularly high quantities of water, so the water removal will necessitate significant energy usage.
The present inventors have sought to further improve the manufacture of alkylene glycol from an alkene. In particular, the present inventors have sought to provide a process that reduces the capital costs and/or the running costs of a plant, whilst ensuring high selectivity.